A wide variety of applied products are devised by forming an oxide superconductor in thick film form on an oxide substrate or oxide base of MgO, alumina, YSZ, or the like or on a metal substrate or metal base of Ag, Au, Pt, Ni, or the like, and causing the oxide superconductor to function. Moreover, an oxide superconductor with a high mechanical strength can be obtained by forming an oxide superconductor on such a base in thick film form.
The following method is tried as a method of forming an oxide superconductor thick film on these types of substrate or base. An oxide superconductor powder to which an appropriate organic binder is added is processed into paste form, thereby obtaining an oxide superconductor thick film manufacturing paste. This oxide superconductor thick film manufacturing paste is applied to the substrate or base using a screen printing method, a doctor blade method, a spraying method, or the like, and then burned. As a result, a polycrystalline oxide superconductor thick film is formed.
This method of forming the oxide superconductor thick film by using the oxide superconductor thick film manufacturing paste does not require an expensive single-crystal substrate and a large-scale, expensive device which needs a high-vacuum system as represented by PVD, CVD, and the like. Therefore, this method is very economical and advantageous in terms of manufacturing cost, and can be considered as a method closest to practical use.
When the case of applying this oxide superconductor thick film to a practical product is taken into consideration, from two viewpoints, i.e., a required superconducting property and a manufacturing cost including a raw material to a manufacturing process, it is regarded as desirable to use a Bi2223 thick film having composition (Bi, Pb)2+aSr2Ca2Cu3OZ (typically, 0<a<0.5) as an oxide superconductor thick film material (hereafter, this composition is referred to as Bi2223, and a film having the composition Bi2223 is referred to as a Bi2223 thick film, a paste having the composition Bi2223 as a Bi2223 paste, and a powder having the composition Bi2223 as a Bi2223 synthetic powder).
In the case of applying the oxide superconductor thick film to a practical product, it is of course important to have a high critical temperature, but simultaneously a high critical current value (hereafter referred to as Ic) is necessary. It is considered that, according to a relation “Ic=oxide superconductor thick film cross-sectional area×Jc” (where Jc denotes a critical current density), a high Ic of the oxide superconductor thick film can be attained by increasing the Jc or the cross-sectional area of the oxide superconductor thick film.
Patent Document 1 describes that a thick film having a Jc of 5,000 A/cm2 or more can be obtained by forming a Bi2223 thick film having a thickness of 130 μm or more on a substrate.
Also, Patent Document 2 describes that a Bi2212 thick film having composition Bi2Sr2Ca1Cu2OZ (hereafter this composition is referred to as Bi2212, and a film having the composition Bi2212 is referred to as a Bi2212 thick film, a paste having the composition Bi2212 as a Bi2212 paste, and a powder having the composition Bi2212 as a synthetic powder) is formed between a base and a Bi2223 thick film so as to suppress any fracture surface between the Bi2223 thick film and the base and thereby enhance an adhesion strength, to attain a film thickness of 200 μm.
Patent Document 1: Japanese Patent Application Publication No. 2004-26625
Patent Document 2: Japanese Patent Application Publication No. 2004-182570